1. Field of the Invention
The invention relates to a method of feeding a gaseous fuel into a premixing burner which has a main line for the feed of oxygenous gas into at least one mixing zone and/or a combustion zone, at least a first portion of the fuel being fed to the main line, in which it is mixed with oxygenous primary gas inside at least one premixing zone and forms a premixture, as well as to a device for carrying out the method. Such a method and a device for carrying it out have been disclosed, for example, by specification EP-A-0 187 441.
2. Discussion of Background
It is generally known in the case of premixing burners to mix the gaseous fuel and/or the combustion air with one another in two or more stages. In the process, for example, a portion of the air necessary for the complete combustion of the fuel is mixed with the fuel inside a mixing zone and is supplied to a subsequent combustion zone (see, for example, specification EP-A-0 187 441). However, it is also possible to first mix a portion of the fuel with a portion of the combustion air. The ignition normally takes place directly after the discharge of the mixture from the premixing zone, i.e. the composition of the fuel/primary-air mixture is selected in such a way that it lies within the ignition limits The residual air and if need be the residual fuel are fed directly to the flame.
Here, good intermixing of the gas flows is important in order to avoid NOx emissions. Local fluctuations in the fuel/air concentration lead inevitably to higher NOx production.
In the known methods, good intermixing of fuel and air inside the mixing zone is not always guaranteed, e.g. since the length of the mixing zone often has to remain limited for constructional reasons. Flow baffle plates, inter alia, in the mixing zone are used as an aid, which flow baffle plates are intended to promote intermixing of the flow media. However, the fitting of such baffle plates means additional effort during the construction and in addition leads to unwanted pressure losses.
In addition, there is always the risk of flashback within the mixture feedline to the combustion zone and also in the premixing zone. So-called "flame retention baffles" serve as a conventional measure to help solve this problem. The use of such mechanical flame retention baffles again involves additional constructional effort, and there is always a risk left to contend with. In addition, the mechanical flame retention baffles are exposed to extremely high thermal loads, which often leads to the flame retention baffles becoming worn or sometimes breaking loose. These parts could then put other parts located further downstream at risk, such as, for example, turbine blades.
The so-called premixing burners of the double-cone type of construction may be designated as flame-retaining burners which do not need mechanical flame retention baffles. Such double-cone burners are disclosed, for example, by U.S. Pat. No. 4,932,861 to Keller et al. and will be described later with reference to FIGS. 3 and 4. The fuel, gas in this case, is injected in the inlet gaps via a row of injector nozzles into the flow of combustion air coming from the compressor. As a rule, these injector nozzles are uniformly distributed over the entire gap.
In order to achieve reliable ignition of the mixture in the downstream combustion chamber and satisfactory burn-up, intimate mixing of the fuel with the air is necessary. Good intermixing also helps to avoid so-called hot spots in the combustion chamber, which lead, inter alia, to the formation of unwanted NO.sub.x.
It would also be possible in principle for a portion of the air required for the combustion to be fed into the fuel line upstream in order to achieve a type of premixing along the fuel line. However, in many applications of the premixing burners, such as, for example, inside the combustion chambers of gas turbines, the pressure in the air line is considerably lower than the pressure in the fuel line. Consequently, the combustion air would have to be highly compressed again during such feeding, which would mean considerably increased mechanical complexity. The consequence of this would in turn be that virtually only a small portion of air could be mixed with the fuel in this way--despite the considerable complexity--so that a satisfactory solution to the problem would still not be guaranteed.